Current studies in newborn infants have shown that an increase in lung volume achieved with continuous distending airway pressure (CDAP) did not affect respiratory center output as measured by mask occlusion pressure, although minute ventilation decreased. Magnetometer recordings indicated that the increased lung volume was accounted for by rib cage expansion, so that the diaphragm can maintain its resting configuration. Application of CDAP also resulted in prolongation of expiratory duration while inspiratory duration remained unchanged. These studies will be extended to observe changes in muscle activity with surface electromyograms of the diaghragm, abdomen and intercostal muscles in full term and premature newborn infants during crying, carbon dioxide breathing and increases in lung volume (sighs) and CO2 breathing, CDAP, and 5-breath airway occlusions will be analyzed. In addition, observations of diaghragm movement will be made with ultrasound during changes in lung volume while infants are spontaneously breathing and with mechanical ventilation.